Process-Structure-Property Relationships of Micron Thick Gadolinium Oxide Films Deposited by Reactive Electron Beam-Physical Vapor Deposition (EB-PVD)

Open Access
Author:
Grave, Daniel Aldo
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 29, 2014
Committee Members:
  • Douglas Edward Wolfe, Committee Chair
  • Joshua Alexander Robinson, Dissertation Advisor
  • James Hansell Adair, Committee Member
  • Michael T Lanagan, Committee Member
Keywords:
  • Thin film
  • Gadolinium Oxide
  • Gd2O3
  • neutron detection
  • residual stress
  • phase transition
Abstract:
Gadolinium oxide (Gd2O3) is an attractive material for solid state neutron detection due to gadolinium's high thermal neutron capture cross section. Development of neutron detectors based on Gd2O3 requires sufficiently thick films to ensure neutron absorption. In this dissertation work, the process-structure-property relationships of micron thick Gd2O3 films deposited by reactive electron-beam physical vapor deposition (EB-PVD) were studied. Through a systematic design of experiments, fundamental studies were conducted to determine the effects of processing conditions such as deposition temperature, oxygen flow rate, deposition rate, and substrate material on Gd2O3 film crystallographic phase, texture, morphology, grain size, density, and surface roughness. Films deposited at high rates (> 5 Å / s) were examined via x-ray diffraction (XRD) and Raman spectroscopy. Quantitative phase volume calculations were performed via a Rietveld refinement technique. All films deposited at high rates were found to be fully monoclinic or mixed cubic / monoclinic phase. Generally, increased deposition temperature and increased oxygen flow resulted in increased cubic phase volume. As film thickness increased, monoclinic phase volume increased. Grazing incidence x-ray diffraction (GIXRD) depth profiling analysis showed that cubic phase was only present under large incidence angle (large penetration depth) measurements, and after a certain point, only monoclinic phase was grown. This was confirmed by transmission electron microscopy (TEM) analysis with selected area diffraction (SAD). Deposition of a pure cubic phase film was possible at high deposition rates for film thickness less than 50 nm under a deposition temperature of 250 °C and 200 sccm O2 flow. However, this film was shown to be under significant compressive stress of 1.7 GPa by XRD residual stress measurements. Thick (1 μm) pure cubic phase films were deposited by lowering the deposition rate to < 1 Å / s, and increasing the deposition temperature to 650 °C. This was attributed to both increased adatom mobility as well as reduction in compressive stress as measured by the wafer curvature technique. Based on this information, a large compressive stress was hypothesized to cause the formation of the monoclinic phase and this hypothesis was confirmed by demonstrating the existence of a stress induced phase transition. An experiment was designed to introduce compressive stress into the Gd2O3 films via ion beam assisted deposition (IBAD). This allowed for systematic increase in compressive stress while keeping a large adatom diffusion length on the film surface. It was shown that the films could be completely transformed from the cubic to monoclinic phase with applied compressive stress of -777 MPa for the deposition conditions considered, confirming the existence of a stress-induced phase transition. Crystallographic texture evolution in the Gd2O3 films was investigated for different substrate types. At high rates, it was shown that films deposited on different substrates (quartz, silicon, sapphire, and GaN) all had similar θ-2θ diffraction patterns, suggesting that films grew similarly on different substrates due to the low adatom mobility. However, significant differences in texture were observed for films deposited at low rates (< 1 Å / s) and high temperature (650 °C) on different substrates. For evaluation of in-plane texture in the Gd2O3 films, pole figure analysis was performed. Mixed phase films deposited at high rates and low temperature showed weak out-of-plane texture and random in-plane texture. Mixed phase films deposited at high temperatures possessed a fiber texture (strong out-of-plane texture), but lacked the necessary adatom mobility to develop in-plane texture. For single phase cubic films grown under low rates of deposition, out-of-plane texture was observed on quartz substrates. However, weak and strong in-plane textures were observed for sapphire and GaN substrates, respectively. The use of ion bombardment resulted in the formation of moderate biaxial texture for films grown on quartz. For films grown on sapphire, a very strong biaxial texture was achieved with ion bombardment which adds additional energy to the system. The effects of processing on the structure, composition, and interfacial chemistry of the Gd2O3 films were investigated. The results showed that films primarily adhered to the Structure-Zone models with a few exceptions. The deviation from the Structure-Zone model was explained by the combined effects of columnar growth, shadowing, and adatom mobility. At low deposition temperatures, decreasing oxygen flow resulted in increased film density due to higher adatom mobility. Films deposited at this temperature were characterized by small (10 – 15 nm) nanocrystalline grains with some porous disordered regions. As the deposition temperature was raised to 650 °C, a dense nucleation region was observed and the films were highly crystalline with larger grain size. After the approximately 200 nm dense nucleation region, columnar growth was observed as the film grew thicker. Porosity formed between the growing columns due to shadowing effects. The effects of processing on the interfacial layer size of Gd2O3 films deposited on silicon were investigated. The size and structure of the interfacial layer was dependent upon the deposition conditions. Generally, higher oxygen flow and higher deposition temperature led to larger interfacial layer size and a more undulating interface. The interfacial reaction was determined to proceed as a result of intermixing of the SiOx layer with the Gd2O3 layer to form the Gd-Si-O interface during deposition. Films deposited on GaN possessed excellent interfacial characteristics with near heteroepitaxial growth and no apparent interfacial layer. Ion beam assisted deposition was shown to result in significant densification of the film structure and elimination of the columnar morphology. Lastly, the O : Gd ratio of the Gd2O3 films was investigated using EDS and XPS, and showed that the processing conditions had significant effects on the film composition. Increasing temperature was found to decrease the O : Gd ratio. Increasing oxygen flow was found to increase the O : Gd ratio for films deposited at low temperatures, but had little effect for films deposited at high temperature due to the fact that adatom mobility was significantly more affected by oxygen flow at lower temperatures. The dielectric properties of Si(111) / Gd2O3 / Ti / Au MOS capacitors were investigated. Moisture absorption in Gd2O3 films was found to result in both increased dielectric loss (10x) and inflated dielectric constant values (~40 %). Heat treatment of the films at 100 °C resulted in outgassing of moisture, reduction in dielectric constant, and excellent frequency dispersion of the dielectric constant over a range of 10 kHz – 1 MHz. The effect of film processing on the dielectric constant was systematically investigated. Tuning of the dielectric constant from a value of 11 to a value of 24 was possible by manipulating the structure and crystallographic phase of the material via the processing conditions. Capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics of GaN / AlGaN / Gd2O3 / Ti / Au MOS capacitors were investigated. The effects of processing on fixed oxide charge, trapped oxide charge, and density of interface states were evaluated. Single phase cubic films deposited at low rates with near heteroepitaxial growth were shown to have the lowest density of trapped charge. Additionally, threshold instability in the C-V curve as well as a non-volatile charge trapping effect was observed. Conduction mechanisms in Gd2O3 films were studied through current-voltage-temperature (I-V-T) measurements. A number of mechanisms were found to govern conduction in Gd2O3 films across a wide measurement temperature and electric field range including ohmic conduction, Schottky Emission, Poole-Frenkel emission, and space charge limited conduction. For Poole-Frenkel emission, the trap heights of 1.03 eV and 1.25 eV were found for the cases of gate and substrate injection, respectively. Schottky barrier height at the Si (111) / Gd2O3 interface and Gd2O3 / Ti / Au interface was found to be 0.56 eV and 0.54 eV, significantly lower than the expected barrier height value. For GaN / AlGaN / Gd2O3 / Ti / Au capacitors, the Schottky barrier height at the GaN / AlGaN / Gd2O3 interface was found to be 0.79 eV and the Poole Frenkel trap height was 0.46 eV. The performance of GaN / AlGaN / Gd2O3 / Ti / Au high electron mobility transistors (MOS-HEMTs) was investigated with emphasis on necessary device performance for radiation detection. Gd2O3 films deposited at 650 °C led to devices with good switching characteristics including an on/off ratio of ~109 and subthreshold swing as low as 88 mV. However, significant hysteresis and threshold voltage instability was observed for the Gd2O3 MOS-HEMTs. Finally, the response of GaN / AlGaN / Gd2O3 / MOS-HEMTs to gamma and neutron radiation was investigated. The devices showed sensitivity to both types of radiation. For devices with 1 μm thick Gd2O3 films, the neutron response was not improved over the control device. Devices with 10 μm thick films showed significantly increased neutron detection. However, the neutron response was larger than the expected theory, suggesting that added signal due to γ-radiation may have been collected.